The Haefeli test is a method to derive the soil permeability coefficient. It is carried out inside an excavation shaped as an inverted truncated pyramid with a square base.
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The dimensions of the excavation are 1.5 x 1.5 m on the surface and 0.5 x 0.5 m at the base; the latter will be excavated 0.5 m below the ground level. Operationally, the excavation is filled with water, one waits for the water to infiltrate the soil, and the flow rate required to maintain a constant level is evaluated. The height of the water column will be set at the operator’s discretion.
The relation used to estimate the permeability coefficient is:
k=Q/[B2 · ( 27·H/B+ α )]
where:
Q is the flow rate in (cm3/s),
B (cm) is the width of the lower base,
H (cm) is the water level above the lower base, and
α is the capillarity coefficient, set equal to 3.